The illness first rose to attention in Brazil in April 2015 after the discovery of an epidemic of a birth defect in which children were born with smaller than normal cerebrums. Since then, the virus has been been found in more than 20 countries in Latin America alone. In January 2016, the CDC released an extensive list of countries pregnant women should not travel to in order to prevent contracting the illness.

But here's why Americans don't need to be fearful that Zika will plague the U.S. just yet: Everyone that has been diagnosed with a Zika infection contracted it while traveling abroad. There have been no cases in which the virus has been transmitted locally.

"We're expecting a lot of travel-associated cases," Anne Schuchat, principal deputy director for the CDC, said on Thursday.

Another thing to consider: The virus is spread by Aedes mosquitoes, which are mostly found in tropical and subtropical regions. In the countries where Zika is spreading rapidly, people can contract the virus in their homes, where the mosquitoes like to dwell. Many Latin American homes don't have air-conditioning; people leave their windows people for fresh air, an invitation for the insects. Plus, more trash in Latin American cities means more opportunities for standing water to collect, creating the perfect breeding grounds for the mosquitoes. In the U.S., most people have screens on windows and doors and air-conditioning.

"(The Aedes mosquitoes) is a bit of a homebody," infectious disease expert Daniel Lucey of Georgetown University told BuzzFeed News.

According to Gonzalo Vazquez-Prokopec, an assistant professor in the department of environmental sciences at Emory College in Atlanta, it's relatively difficult for individual travelers infected abroad to spread Zika in the U.S. The primary reason is because most people infected with the virus clear it from their blood in less than a week. Mosquitoes can only become infected with the virus if they bite someone during that small window of time. Many travelers have cleared the virus before they even return to the U.S., Vazquez-Prokopec reported.

What's more, the mosquito doesn’t travel far from where it’s born, a limiting factor in its ability to spread the Zika virus. Moreover, in the U.S., most swamps are drained, eliminating a popular place where the bug lays eggs.

At a press conference on Thursday, American officials insisted that a Zika outbreak in the U.S. is unlikely and reminded people that the country’s history of mosquito-control efforts have previously suppressed other mosquito-borne illnesses such as dengue and chikungunya.